Photosynthesis is the biologyprocess by which plants make food or sugar. In simple terms, it is also known as carbon fixation because basically, carbon in the form of CO2 bonds with H2O (water) to form CH2O (sugar) and O2 (oxygen) in the presence of solar energy. Sugar formed in this reaction is the raw material for producing other complex organic compounds. Photosynthesis is basically of 3 types – C3, C4 and CAM. C3 photosynthesis is called so because CO2 is first fixed into a 3 carbon compound in the presence of RUBISCO enzyme. This is the major form of photosynthesis that most plants undertake. C4 or CAM are the adaptations to survive in conditions where C3 turns out to be wasteful than beneficial for plants. Like in arid conditions, plants adopt C4 cycle for efficient water usage. C4 plants can photosynthesize at a faster rate under extreme heat and light conditions as they adopt an extra biochemical pathway and special leaf anatomical features to reduce loss of water.
In C4 pathway, CO2 is first incorporated into a 4 carbon compound. These plants adopt an efficient way to deliver CO2 to RUBISCO enzyme. These plants adopt a specialized leaf anatomy known as kranz anatomy. Here, chloroplasts exist in mesophyll as well as bundle sheath cells. So, this is achieved by using a efficient enzyme i.e. PEP carboxylase to fix CO2 in mesophyll cells and then this fixed carbon is transported to bundle sheath cells via aspartate or malate. In the bundle sheath cells, RUBISCO is isolated from atmospheric oxygen and saturated with CO2 released by decarboxylating malate or oxaloacetate. This localized CO2 enrichment within the bundle sheath cells leads to higher rates of photosynthesis. As this process utilizes extra energy, C4 plants efficiently fix carbon in only certain conditions. PEP carboxylase shuttles CO2 at a faster rate because of which stomata are not open for a very long time and thus leads to less transpiration water loss. In other words, the rate of dark respiration is less and light compensation point is always lowered.
This pathway was discovered by Marshall Davidson Hatch and C R Slack in 1966, so it is also called as Hatch Slack pathway. Plants possessing this ability to utilize C4 pathway have a competitive advantage over other plants which use C3 pathway. This pathway gives them an immunity to survive over other plants in harsh temperature, light, drought or CO2 or NO2 limitation conditions. C4 plants include several thousand species in at least 19 families of plants. They are less common in dicots than in monocots. Mostly all are angiosperms. Examples are fourwing saltbush, corn, maize, sorghum, sugarcane, etc. Many scientists across the world are working towards conversion of C3 plants to C4 because of the above mentioned added advantages of this pathway. One of the ongoing projects is the C4 rice project, as rice is the staple food of many countries.
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